fryke, I'm afraid I have to disagree. Most of those examples just show that hardware gets better over time.
One of them is a particularly special circumstance: the processor change, when ppc code was poorly optimized, even when you were lucky enough to have it at all. That's a far more drastic change than anything we're discussing here.
And one of them is very very different from my experiences: the assertion that the g3s were slower than g2s. My only question about the original g3 launch was why anyone would every buy anything other than the lowest-end model, given how incomprehensibly fast even those were. I still think of the original g3 release as being the peak of the ppc architecture, when its lead over competing systems was most dramatic.
I think nearly every mac I've owned has been the first model after a large upgrade, and I've been happy with every one of them: the first g4, the first quicksilver, the first g3, the first powerbook g3 series, and the powerbook 540 (you listed it as being the last of the previous line, which rather overlooks that it was also the first of that line, which I don't think ever saw any revisions).